Diatomaceous earth (siliceous earth, diatomite) is a mineral mainly comprising of amorphous silicone dioxide having a natural colloidal structure and high specific surface area originating from the fossile frustules (the hard and porous cell wall or external layer) of diatomaceous algae (Diatoms). Besides the frustules of diatomaceous algae, diatomaceous earth contains further minerals in various amounts (e.g. montmorillonite, caolinite, quartz, feldspar, calcite). Diatomaceous earth is a loamy, earthy, soft, easily dispersible, fine-grained, usually light-coloured silicon-containing sedimentary mineral. It is of natural origin formed from the bulk deposition of the frustules of fossile Diatoms in salty and freshwater lakes and seas of the early Jurassic period. Many occurances of the diatomaceous earth are known worldwide.
Diatomaceous earth is used in various fields of the industry. It has been used for a long time in the chemical, cosmetical and pharmaceutical industry as a general auxiliary agent having high sorption capacity and relative lack of chemical reactivity.
During the use in the chemical industry, high specific surface area of diatomaceous earth particles is exploited. Although specific surface area of artificial amorphous silicone dioxide derivatives (e.g. Aerosil) is similarly high, diatomaceous earth exhibit far more advantageous properties during pharmaceutical formulation methods involving compression than said artificial silicaceous products. Diatomaceous earth has favourable pore structure comprising macro- meso- and micropores, which provides advantageous wetting and adsorption properties during the formulation of hydrophilic and hydrophobic materials or mixtures including emulsions or colloidal systems.
Since diatomaceous earth is a naturally occuring material of organic origin, a critical characteristic of the product is its purity. Diatomaceous earth is produced by mining the natural deposit, separating, calcining and chemically purifying the mineral. (Lloyd de Antonides 1998, Diatomite U.S. Geological Survey Mineral Commodity Summaries 1998; Tasnády Kubacska András: Ásványok).
A common feature of silicon-containing preparations used in the pharmaceutical industry is that these materials are purified by calcination. Calcined diatomaceous earth has been admitted into the United States Pharmacopoeia. The principle of the manufacturing method disclosed in the pharmacopoeial monograph resides in separating crude diatomaceous earth in an air jet mill or by sieving, separating silicon-containing part from carbonates by acidic treatment and heat-treating (calcining) the resulting material at a high temperature (calcination at 900-1100° C.). During the heat treatment, volatile impurities are vaporized and thereby removed and the solid material partially melts, thus forming beads. The treated material is milled and the acidic treatment is optionally repeated in order to remove acid-soluble components. The diatomaceous earth powder is dried and sized. While this method yields a product having adequate chemical purity, the natural structure of diatomaceous earth originating from diatomaceous algae is broken, thus destroying the greater part of macro- and mesoporous structure.
There are no known purification methods in the prior art which are devoid of calcination step. For example, in published U.S. Pat. No. 2,164,500, the calcining step is essential. Published U.S. Pat. No. 4,325,844 discloses an energy-efficient method for calcination, which involves the treatment of diatomaceous earth at a temperature which is significantly higher than that applied in method of the present invention.
Chinese Patent Application No. 100346858 is related to a method for purification of diatomaceous earth and a process for removing lubricant oil therefrom, which comprises wetting of diatomaceous earth, treating the wetted solid with an acid, preferably using sulfuric acid and filtering, neutralizing and dehydrating the solids.
Chinese Patent Application No. 1401567 is related to purification of diatomaceous earth by acidic treatment using oxalate or phosphate, thereafter thoroughly stirring, ultrasonically treating or boiling the mixture. Subsequently the solids are allowed to settle and the iron-containing sediment is removed magnetically.
Chinese Patent Application No. 1322673 is related to a two-step method for the purification of diatomaceous earth. In the first step, the structure of the inorganic impurities is transformed by heat-treatment. Subsequently in the second step, the material is treated with sulfuric acid while heating at low temperature until a purified silicone dioxide product is obtained.
Published Japanese Patent Application No. 2001097711 discloses a method for purification of diatomaceous earth free of boron impurity and suitable for manufacturing silicone semiconductors, which comprises the steps of acidic treatment optionally followed by neutralization with a basic reagent; heating and separating diatomaceous earth at a suitable pH and optionally treating diatomaceous earth with a suitable salt, for example, aluminium nitrate.